Touched

 Jeremy finds himself in the unfortunate position of being dead. A ghost.

Presented with the facts of afterlife, with no heaven or hell to worry about, he discovers that he, and every other person that ever lived (and not a few animals), is immortal. That life as a ghost is real, and obeys certain rules that he learns as he goes. The real world continues around them, unaware of the universe of ghosts around them. This would be great if they could touch anything. They can't even touch themselves, and certainly not any other ghosts. Sure, they can fly to the Moon, or visit Europa with its alien ghosts beneath the ice. But no smell. No taste. No touch. Hardly a paradise, and that's why so many ghosts let themselves fade away.

But all this changes when Jeremy fights the ghostly monsters on Mars left over from millions of years ago, when Mars was still teeming with life. And as a result, ghost life will never be the same.